In modern rolling mills, there are a variety of differing rolling processes and procedures for producing finished and semi-finished metal products. Typically, heated slabs or billets (of steel or aluminum, for example), produced by ingot pouring or continuous casting machines, are hot rolled through one or more mill roll stands to produce elongated fininshed or semi-finished products such as plates, sheet, strip, bars, rods, structural shapes and the like. Further finishing steps may include cold rolling, such as the cold rolling of hot strip to cold rolled sheet and strip products. Such roll stands generally comprise at least one pair of work rolls between which the metal workpiece is passed to reduce the thickness dimension and/or to shape the metal workpiece as desired. The work rolls are provided with backup rolls.
During the metal rolling operation, mill rolls are heated by a work heat due to the plastic deformation of the rolled metal, a frictional heat generated between the rolled metal and the work rolls, and, in case of hot rolling, heat transfer from the hot metal workpiece. Particularly in the case of hot rolling steel, where a steel workpiece to be rolled is preheated to temperatures in excess of 1200.degree. C. roll heating as a result of heat transfer can become excesssive.
Because of such roll heating, it is essential in practically all metal rolling operations that means be provided to cool the rolls during use and thereby prevent thermal expansion of the rolls and roll surface deterioration, each of which can adversely affect the quality of the rolled product and reduce the service life of the rolls.
While numerous differing types of apparatus have been utilized to cool rolling mill rolls, most have been based on the provision of a line of coolant nozzles spaced along a side surface of the roll parallel to the roll axis, and positioned on either or both the entrance and/or the exit side of the roll. Typically, an elongated spray bar, i.e. a manifold or header, having a width substantially equal to the width of the roll, is closely positioned parallel to the roll. The spray bar is provided with a plurality of spaced spray nozzles to direct the water or other coolant from the manifold onto the rotating roll, generally with a nozzle pressure of less than about 100 pounds per square inch (psi) (7 bars). Normally, one or several such spray bars are provided having a fixed flow rate and pressure as will achieve an optimum cooling rate, i.e. a cooling rate as necessary to keep the roll temperature within determined temperature limits. Since the roll surface is heated quite significantly and very rapidly at the roll bite, i.e. while in contact with and working the heated workpiece, the prior art practice has been to utilize a coolant flow rate and pressure as will cool the roll surface as significantly and nearly as rapidly as it is heated. Hence, the more rapidly and excessively a roll becomes heated, the more rapidly and excessively it is cooled. This necessitates the provision of one or more pumps of relatively high fluid pumping capacity.
Particularly in the case of hot rolling steel products, wherein workpiece temperatures normally are in excess of 1200.degree. C., and utilizing conventional roll cooling apparatus, roll surface deterioration can progress rather rapidly as a result of "firecracking," i.e. cracks occuring in the roll surface resulting from thermal fatigue stresses, which thereafter leads to roll "banding," i.e. bands of lost surface metal occurring circumferentially around the roll surface. These phenomena not only result in a reduced surface quality of the rolled product, but also cause mill scale and other oxides dislodged from the roll surface to be rolled in the surface of the workpiece to further reduce the product's surface quality.
Based on numerous studies, the surface deterioration of hot rolling mill rolls, as a result of thermal fatigue stresses, has been found to be characterized by the following sequence of events:
a. Formation of thermal fatigue cracks.
The formation of fine thermal fatigue cracks in the surface of a rolling mill roll will start to form after only a few revolutions of the roll. These cracks tend to form and grow primarily in a direction perpendicular to the roll surface and to penetrate to a depth governed by the magnitude of temperature gradients caused by the very rapid alternate heating and cooling of the roll during a rolling operation. In rolls subjected to a heavy thermal load and good cooling, the crack depth may approximate 250 micrometers (0.010 inch). The cracks will grow and intersect to form cells of various sizes on the roll surface. For example, near the outer surface of the rolls, where temperature gradients are very high, a very fine crack network will be effected with cells having side dimensions of 25 to 50 micrometers (0.001 to 0.002 inch. The deeper cracks will define larger cells having side dimensions of 250 to 500 micrometers (0.010 to 0.020 inch).
b. Formation of subsurface cracks.
Subsurface cracking will occur simultaneously with surface cracking at depths of 250 micrometers (0.010 inch) beneath the roll surface and oriented parallel to the roll surface. While these subsurface cracks could be thermal in origin, it is believed that they ore more likely caused by the mechanical stresses resulting from the pressures imposed by the workpiece and backup roll. The number of these horizontal cracks will decrease at increased depth below the surface.
c. Oxidation of crack surfaces.
The interfaces of both the surface and subsurface cracks, being exposed to highly corrosive gases or liquids, such as cooling water and steam, quickly form oxides within the interface gaps.
d. Growth of defective areas.
Because the oxide formations will grow and thicken within the crack interfaces, additional stresses will be imposed at the surface of the roll, producing a wedging action on the cells similar to that in stress corrosion.
e. Removal of the roll cells.
In advanced stages, pieces of the roll surface become completely surrounded by oxide products and are either removed from the surface of the roll by the rolling mechanical forces or by the wedging action of the oxide products. These loosened particles may be rolled back into the roll surface or may be rolled into the surface of the workpiece as mill scale. The plastic flow of roll particles near the surface (within 25 micrometers (0.001 inch) indicates the presence of very high shear stresses which will dislodge cells that are loosely attached.
f. Banding
The banding process is a macro deterioration of the roll surface that results from the removal of many close or side-by-side cells, often resulting in elongated bands of removed surface cells. The bands normally propogate circumferentially because the surface shear stresses due to rolling are circumferentially imposed, and because the loss of neighboring cells will reduce attachment forces and facilitate additional cell removal at the edge.
The oxide surface that is found on the roll surface, while it is still thin, plays a positive role by protecting the actual roll metal surface from thermal shocks. Therefore, there is an optimum thickness of the oxide layer on a roll that should be maintained. The present method and apparatus partially remove the oxide layer, leaving some oxide for the aforesaid protective purposes.